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Don’t panic about the missing Higgs – for now

The world’s most-wanted particle continues to elude the world’s most powerful particle accelerator. A sign that the elusive Higgs boson doesn’t exist? Not so fast. For now, there are good reasons to assume the Higgs is just hiding.

“It’s never too early to think about it, but it is too early to worry,” says Nobel prize-winner Frank Wilczek of the Massachusetts Institute of Technology.

The still-hypothetical Higgs boson is thought to endow all other particles with mass. Confirming its existence would complete the standard model of physics, the leading theory for how particles and forces interact. Finding the Higgs and pinning down its mass, or ruling out its existence and paving the way for new models, is one of the goals of the Large Hadron Collider at CERN near Geneva, Switzerland.

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Since it started smashing protons together in 2009, the LHC has steadily collected data that help rule out various masses for the Higgs from a range of possibilities allowed by the standard model. Combined with earlier results from other accelerators, the latest LHC limits, announced on 22 August at the Lepton-Photon conference in Mumbai, India, mean the Higgs is now restricted to having a mass of between 115 and 145 gigaelectronvolts (GeV), or 122 to 154 times the mass of a proton (mass and energy can be treated interchangeably for particles).

Low-hanging fruit

This means that about 90 per cent of the possible masses for the Higgs are now ruled out, says CERN spokesman James Gillies.

And that has sparked speculation that the Higgs might not exist at all. If this turns out to be true – and it’s certainly a possibility – it would leave room for exotic theories involving an extra force of nature. Indeed, some are banking on this. “I don’t pay close attention to it, because I don’t think it’s going to show up anyway,” says Ken Lane of Boston University.

It is still too soon to assume the universe is Higgs-less, however. Take the small mass range that’s now left. It is at the low end of potential masses and so is the most difficult for the LHC to explore. A heavier Higgs would leave clearer tracks in the detector, but the lighter it is, the harder it would be to tell a true signal from other particles that produce similar tracks.

“In the low mass range, disentangling signals from noise is a more painstaking process, and needs more data,” Gillies says. In other words, rather than sounding a death knell for the Higgs, the latest limits could merely signal that we have sorted through the low-hanging fruit.

Strict supersymmetry

Some Higgs hunters are actually relieved that it hasn’t shown up yet as they never expected the particle to be heavier than 145 GeV. That’s because, as the Higgs gives mass to the known particles, these particles can put limits on the Higgs’s mass. Earlier experiments at accelerators like the Tevatron, the Large Electron Positron Collider and the Stanford Linear Collider suggest that the Higgs should be slightly heavier than the W and Z bosons, at between 120 and 130 GeV.

An extension of the standard model called supersymmetry makes an even stronger case for a light Higgs. Theoretically, the standard model allows the Higgs to weigh almost anything. But supersymmetry, which calls for twice as many particles as the standard model, has stricter rules. This theory directly links the Higgs mass to the W and Z boson masses, plus offers some quantum corrections that bump it up to about 130 GeV.

“In a sense, these [latest] results from the LHC are not surprising,” Wilczek says. “It would have been very embarrassing for supersymmetry and the properties of the particles we know about if the Higgs had been discovered in the range of masses that have [now] been explored.”

There’s another explanation for the Higgs’s absence&colon; it decays into particles we don’t know how to detect, such as dark matter or as yet unknown particles. In that case, the only way to detect the Higgs would be by looking for tiny amounts of missing energy.